Tach pickup on the transmission housing does not go directly to the cluster. It goes in to the EGR controller then out to the cluster. But you said you do not have a EGR controller so this is turning out to be a mystery.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked